With the advent of controlled rectifiers (such as SCR's or thyristors) and variable-frequency d-c/a-c converters, the use of induction motors as a relatively maintenance-free alternative to the d-c shunt motor is gaining increased attention. Thus, in a paper entitled "CURRENT-SOURCE CONVERTER FOR AC MOTOR DEVICES" by Kenneth P. Phillips (IEEE Transactions in Industry Applications, Vol. IX-8, No. 6, November/December 1972, pages 679-683) there has been described a converter circuit which utilizes current and frequency adjustments in order to stabilize the speed of a squirrel-cage motor at a selected level in the face of variable load conditions. A first array of thyristors. responsive to feedback to a speed-error signal from a motor-driven tachometer, converts the line current into a direct current of load-dependent magnitude which is then reconverted by a second thyristor array into a three-phase operating current of a load-dependent frequency determined by a similar feedback signal.
In such a system, sudden variations in loading conditions (such as may occur, for instance, in the event of a break in the transmission between the rotor and the load) are liable to cause abrupt changes in the input voltage of the motor which may have serious consequences for the motor itself and/or for the associated control circuits. These changes result from the somewhat sluggish response of the first thyristor array to the altered torque requirements in re-establishing the proper relationship between slip frequency and current supply. Since the stator current is the vector sum of the load-dependent rotor current and the excitation current required for the generation of the magnetic flux, changes in the former affect the latter and also the input voltage appearing at the phase windings of the stator.